This invention relates to a distillation process for separating C4 olefins from C8 olefins with a bottoms temperature that is lower than what is usually required for effective separation of C4 olefins from C8 olefins.
Processes for the oligomerization of light olefins to produce C8 olefin oligomers are known. Oligomerization processes have been long employed to produce high quality motor fuel from C4 olefins. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Methods have always been sought to improve the C8 olefin number of the gasoline boiling range oligomerization products.
In the oligomerization method of the indirect alkylation process set forth in, for example, U.S. Pat. No. 6,080,903 B1; U.S. Pat. No. 5,990,367 B1 and U.S. Pat. No. 5,895,830 B1, light aliphatic olefins such as C4 olefins are contacted with solid phosphoric acid (SPA) catalyst in the presence of a heavy paraffin diluent such as cyclohexane or octane. The presence of the paraffin diluent is believed to promote the oligomerization in the liquid phase to yield predominantly dimerized oligomers such as C8 olefins. The heavy aliphatic olefins can be saturated to provide high octane fuel. Moreover, light paraffins feed can be dehydrogenated to provide the feed for the indirect alkylation process. Patents disclosing such dehydrogenation include U.S. Pat. No. 4,393,259 B1; U.S. Pat. No. 5,049,360 B1; U.S. Pat. No. 4,749,820 B1; U.S. Pat. No. 4,304,948 B1 and U.S. Pat. No. 2,526,966 B1.
Other oligomerization processes using an ionic exchange resin catalyst to oligomerize light olefins to produce oligomers such as C8 olefins are also known. These processes often include an oxygenate such as tert-butyl alcohol (TBA) or sec-butyl alcohol (SBA) in the feed for modifying the catalyst to maintain desired selectivity. References disclosing resin catalyzed oligomerization include U.S. Pat. No. 5,877,372 B1 and EP 0994088 A1. The resin catalyzed oligomerization can also be preceded by a dehydrogenation zone to convert paraffinic feed into olefinic feed. The oligomerization can also be succeeded by a hydrogenation zone to convert heavy oligomeric olefins into heavy alkanes that can be blended with gasoline stock.
In such oligomerization processes, either before or after hydrogenation, it may be necessary to separate unreacted light olefins from the product heavy oligomers in the effluent from the oligomerization zone. Separation is conventionally performed in a distillation column.
In the distillation column typically after the oligomerization zone the lighter components comprising primarily unreacted light olefins such as C4 olefins and non-participating light alkanes such as C4 alkanes that were present in the feed stream exit from the overhead of the distillation column. The heavier components comprising primarily heavy oligomers such as C8 olefins and smaller amounts of C12 olefins along with a heavy paraffinic diluent, if the oligomerization catalyst is SPA, exit out the bottoms of the distillation column. If oxygenate is part of the feed to the oligomerization zone to moderate the resin catalyst instead of paraffinic diluent for SPA catalyst, the oxygenate may go out either or both of the overhead or the bottoms of the column. Because the heavy components such as C8 olefins have a relatively high boiling point temperature which is much higher than the boiling point temperature of the lighter components such as C4 hydrocarbons, the relatively high bottoms temperature of the distillation column will require a higher temperature heat source to boil the contents of the column. For example, if steam is used as a heating medium, steam will have to be delivered at higher pressures to generate the higher temperature. Hence, heating tubes having a sturdier construction are required to handle the higher pressure steam, thereby requiring more complex design and construction costs.
The necessity for the bottoms reboiler of the distillation column to run at a higher temperature is becoming more salient because of recent governmental regulations to decrease or eliminate the use of methyl tert-butyl ether (MTBE) as a gasoline blending component. Many MTBE producers are seeking to convert their MTBE plants to C4 olefin oligomerization plants. MTBE plants generally include a reactor filled with a resin catalyst followed by a distillation column which separates unreacted C4 olefins from MTBE which is brought out the bottoms. The boiling point temperature of MTBE is much less than the boiling point temperature of C8 olefins. Use of an existing C4 olefin/MTBE distillation column for a C4/C8 olefin distillation column would require a reboiler upgrade so the reboiler could handle the higher temperature operation.
Ways of adding agents to the influent for distillation columns to facilitate separation are known. U.S. Pat. No. 5,100,515 B1 discloses an extractive distillation column which adds a saturated alcohol sulfolane or glycol mixture to an alkane and a close boiling alkene to decrease the volatility of the alkene and facilitate separation. U.S. Pat. No. 5,382,330 B1 discloses adding an agent to a mixture of octane and octene to form an azeotrope between the agent and the octene to decrease the volatility of the octene which is brought out the bottom of the column.
EP 0994088 A1 discloses a distillation column with extra C6 hydrocarbons in the feed stream, but the predominant olefins are C5 olefins, not C4 olefins, and the disclosure does not teach reducing the bottoms temperature.
A creative way of dealing with unreacted butenes from an oligomerization effluent is disclosed in U.S. Pat. No. 6,025,533 B1. This patent discloses a distillation column in an oligomerization process that has a catalytic distillation section for oligomerizing unreacted olefins in the column.
An object of the present invention is to reduce the reboiler temperature needed for separating an oligomer from its unreacted olefin in a distillation column.
A further object of the invention is to reduce the reboiler temperature of a distillation column for separating unreacted C4 components from product C8 components.
A simple but effective solution for reducing the reboiler temperature required in a distillation column for separating unreacted lighter olefinic components from heavier oligomeric components has been discovered. The solution requires no modification to the column or the reboiler. Instead, an intermediary with a boiling point between those of the light components and the heavy components is added to the feed to the distillation column. For example, if the heavy components are C8 hydrocarbons and the lighter components are C4 hydrocarbons, C5 or C6 hydrocarbons may be used. Typical refiners have many sources of such materials on site. A predetermined amount of such C5 or C6 hydrocarbon intermediary may be supplemented to the oligomerization feed or effluent and be taken out the bottom of the distillation column with the heavy components. All the heavy components will go out the bottom of the distillation column and the distillative separation will then be effected between the C4 components and the C5 and/or C6 components. Accordingly, a higher temperature will not be required of the reboiler in the bottoms of the distillation column. Lastly, most of the added intermediary component would eventually be directed to the gasoline blending pool of a refinery anyway. Hence, the mere diversion of the intermediary through the column will not significantly change the composition of the refinery gasoline pool or substantially add to the cost of operating the unit.
In one embodiment, the present invention relates to a process for separating C4 olefin from C8 olefin by distillation. The process comprises feeding a mixture of C4 olefin and C8 olefin to a distillation column. An intermediary comprising a hydrocarbon having a boiling point that is greater than a boiling point of C4 olefin and less than a boiling point of C8 olefin at the same conditions is fed to the distillation column. An overhead distillate product containing a smaller volume percentage of the C8 olefin and the intermediary and a larger volume percentage of the C4 olefin than contained in the mixture is withdrawn. A bottoms product containing a larger volume percentage of the C8 olefin and the intermediary and a smaller volume percentage of the C4 olefin than contained in the mixture is withdrawn. Lastly, the process is performed with a bottoms temperature that is less than 300xc2x0 F. (149xc2x0 C.).
In another embodiment, the present invention relates to a process for separating C4 olefin from C8 olefin by distillation. The process comprises feeding a mixture of the C4 olefin and C8 olefin to a distillation column. An intermediary comprising C5 hydrocarbons is fed to the distillation column. The intermediary has a concentration in the mixture that is less than the concentration of C4 olefin in the mixture. An overhead distillate product containing a smaller volume percentage of the C8 olefin and the intermediary and a larger volume percentage of the C4 olefin than contained in the mixture is withdrawn. A bottoms product containing a larger volume percentage of the C8 olefin and the intermediary and a smaller volume percentage of the C4 olefin than contained in the mixture is also withdrawn.
In a further embodiment, the present invention relates to a process for separating a monomer from an oligomer of the monomer by distillation. The process comprises feeding a mixture of the monomer and the oligomer to a distillation column. C4 olefin is the predominant monomer and C8 olefin is the predominant oligomer. An intermediary comprising a hydrocarbon having a carbon number that is greater than a carbon number of the predominant monomer and less than a carbon number of the predominant oligomer is fed to the distillation column. An overhead distillate product containing a smaller volume percentage of the predominant oligomer and the intermediary and a larger volume percentage of the predominant monomer than contained in the mixture is withdrawn. A bottoms product containing a larger volume percentage of the predominant oligomer and the intermediary and a smaller volume percentage of the predominant monomer than contained in the mixture is also withdrawn.
Additional objects, embodiments and details of this invention can be obtained from the following detailed description of the invention.